issue of default frame_id for velodyne_pointcloud/Transform2

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I am testing loading a pcap file and transform velodyne_msgs/VelodyneScan into /velodyne/pointcloud2, and that the converted pointcloud2 data looks not right. And I think that this is caused by the default frame_id setting.

By default, the frame_id of velodyne_msgs/VelodyneScan is "velodyne" and the frame_id for /velodyne/pointcloud2 is "odom", and here is my launch file,

<launch>
  <node pkg="tf" type="static_transform_publisher" name="velodyne_broadcaster" 
        args="0 0 2 0 0 0 base velodyne 10" />
  <node pkg="tf" type="static_transform_publisher" name="base_broadcaster"
        args="0 0 0 0 0 0 odom base 100" />
  <node pkg="tf" type="static_transform_publisher" name="odom_broadcaster"
        args="0 0 0 0 0 0 map odom 100" />

  <node pkg="velodyne_common" type="read" name="read" 
        args="-f $(find velodyne_common)/../velodyne_data_01/unit_46_Monterey_subset.pcap" >
        <param name="frame_id" type="string" value="velodyne" />
  </node>

  <!-- start nodelet manager -->
  <node pkg="nodelet" type="nodelet" name="velodyne_nodelet_manager" 
        args="manager" />

  <!--transform velodyne/packets into pointcloud2 via nodelet -->
  <node pkg="nodelet" type="nodelet" name="transform2" 
        args="load velodyne_pointcloud/Transform2 velodyne_nodelet_manager" > 
  </node>

  <node pkg="rviz" type="rviz" name="rviz" />

</launch>

The original data coordinates are relative to the velodyne frame, yet the converted pointcloud2 shifted down (2 meters, in this case) from the velodyne frame. Then I have to respecify the frame_id for pointcloud2 to "velodyne" in order to locate the resultant pointcloud2 correctly.

So my feeling is that the above default frame_id looks not so reasonable, since typically there is a determinate tf tree in the scenario and the pointcloud2 should not be remapped arbitrarily to a frame other than the default value "velodyne". Correct me if I am wrong.

clark